1. Field of the Invention
This invention relates to a light beam scanning apparatus and a method of correcting unevenness in the scanning lines in the sub-scanning direction in a light beam scanning apparatus having a mechanical light deflector such as a rotating polygonal mirror rotating about a rotational axis.
2. Description of the Prior Art
A mechanical light deflector such as a rotating polygonal mirror, a galvanometer mirror, a bimorph mirror or a hologram scanner rotating about a rotational axis has been employed in a light beam scanning apparatus for scanning a recording medium with a light beam in order to record information on the recording medium or to read out information recorded thereon. In the light beam scanning apparatus the scanning lines are deviated in the direction of the sub-scanning direction due to periodic change in the inclination of the deflecting surface and/or nonperiodic change of the deflecting surface generated by deflection of the rotational axis, thereby causing unevenness in the scanning lines. Unevenness in the scanning lines lowers accuracy in writing information and/or accuracy in reading information.
It has been proposed to prevent the periodic and nonperiodic unevenness in the scanning lines by use of various special optical systems. For example, in Japanese Unexamined Patent Publication No. 54(1979)-69450, it is proposed to lower the angular magnification by use of a diffraction grating. In Japanese Unexamined Patent Publication Nos. 57(1982)-20715, 57(1982)-210315 and 51(1976)-6563, and U.S. Pat. No. 3,897,132, it is proposed to oppose a pair of reflecting mirrors combined to extend perpendicular to each other and to form an edge perpendicular to the rotational axis of the rotating polygonal mirror against the reflecting surface of the polygonal mirror so that the light beam is once reciprocated between the rotating polygonal mirror and the pair of reflecting mirrors. In Japanese Unexamined Patent Publication No. 58(1983)-93021, it is proposed to insert an f.theta. lens system consisting of two groups including a cylindrical lens between a rotating polygonal mirror and a surface to be scanned.
In the former two cases, there is a problem that a correcting optical system must be inserted between the rotating polygonal mirror and the convergent lens (f.theta. lens) to elongate the optical path between the polygonal mirror and the convergent lens, thereby enlarging the convergent lens and the overall scanning apparatus. In the last case in which an f.theta. lens system including a cylindrical lens is employed, it is difficult to adjust lenses and the lens system on the side of the surface-to-be-scanned is substantially enlarged.
There also has been known a method of preventing unevenness in the scanning lines by use of a correcting deflector without use of a special optical system. For example, in the system disclosed in Japanese Unexamined Patent Publication Nos. 53(1978)-146443 and 55(1980)-15197, deviation of the scanning lines in the sub-scanning direction is detected before the scanning starting point and the correcting amount of the correcting deflector is determined on the basis of the detected deviation. In the system disclosed in Japanese Unexamined Patent Publication No. 57(1982)-150817, correction amounts corresponding to the respective reflecting surfaces of the rotating polygonal mirrors are detected in advance and stored, and the correcting deflector corrects the scanning line on the basis of the correction amount read out corresponding to the reflecting surface actually deflecting the light beam. However, though either of the former and latter systems can compensate for the periodical unevenness in the scanning lines, neither of the systems can compensate for the nonperiodic unevenness of the scanning lines.